A wind turbine yaw system is located between the nacelle and the tower of a wind turbine and serves for rotating the rotor of the wind turbine about the tower axis. It typically comprises a gear assembly with a ring gear fixed to the tower and at least one motor driven pinion gear fixed to the nacelle. Usually, at least two motor driven pinion gears are present for realizing a more even load distribution over the ring gear. An example for a wind turbine yaw system is described in WO 2008/053017 A2. When the rotor axis of the wind turbine is not in alignment with the wind direction the yaw system rotates the nacelle such as to bring the rotor axis into alignment with the wind direction by driving the pinion gears which mesh with the ring gear.
As already mentioned, typically at least two drive units and pinion gears meshing with the ring gear are used. However, such a configuration represents mechanically connected motors which can lead to load sharing issues like different loads experienced by the motors not only when motors of different sizes are used but also when motors of the same size are used which have slightly different characteristics. Although at low load the low sharing is usually not an issue, at high or full load the driving motor with lower slip takes a higher share of the load than the motor with a higher slip. This in turn induces unequal tear and wear of the mechanical components of the drive motors and the transmissions.